Name of the website shown with page title in search results

My website is Human Anatomy. According to my knowledge, wordpress is designed to show the name of the website with the title of the pages but I have made some modifications in the header.php file so that the name of the website is not shown. The browsers follow my commands and do not display the name of the website but when I add new pages to my website and search engines index them, they do show the name of the website with the title of the page. For example, I created a page “Esophagus”. The browser shows the title of the page to be “Esophagus” which I want it to show, but when google indexed the page, it displayed it as “Esophagus – Human Anatomy” in its search results.
I cannot figure out what is the exact cause of this.